1. Field of the Invention
The invention relates to a metallic component of a gas turbine installation, such as a turbine blade, which is formed of a nickel-based base material and at least two coating layers superimposed on the base material for improving corrosion-resistance thereof.
Many components exposed to hot gas, especially in gas turbines, are not only subject to thermal, mechanical and erosive stresses but also to corrosive influences to a marked extent. Deposits which form from salts and have an origin which can be traced to fuel and air impurities, lead, together with some gaseous substances, to corrosive damage by high-temperature corrosion (HTC). The causes of corrosion may be quite heterogeneous. On the one hand, the type and source of the fuel and, on the other hand, the composition of the combustion air determine the various forms of attack or aggression which are developed by different chemical mechanisms. In fuels, varying contents of sulfur in natural gases and crude oils, vanadium components in heavy oil, heavy metals in blast-furnace gas, and heavy metals and chlorides in coal gas can exert an influence. In the composition of combustion air, liquid and solid aerosols contained therein play a decisive role; thus, depending upon the site of the installation, the combustion air may contain heavy metals, alkalis and/or chlorides.
2. Description of the Related Art
Various coating layers, including multiple coatings for components exposed to hot gas, have become known heretofore in relatively great numbers for various purposes from the literature. In particular, U.S. Pat. No. 4,123,594 discloses metal objects with a gradated or progressive coating thereon. The innermost layer is a diffusion layer which contains chromium primarily. The gradated coating described in the German application is intended generally to protect the metal object from heat corrosion; in this case, corrosion tests at temperatures of approximately 925.degree. C. are described.
German Published Non-Prosecuted Application 28 26 909 discloses a further double layer for metal objects undergoing such stresses, an inner partial layer thereof having the elements aluminum, chromium and yttrium as constituents. U.S. Pat. No. 3,649,225 also describes double layers which are intended to prevent high-temperature corrosion. In most conventional double layers, the generally thin lower layer does not itself offer protection against external attack but instead merely improves the durability and adhesion of the upper layer.
Conventional layer systems protect a component against oxidation and corrosion at very high temperatures, but intensive tests have shown that the heretofore known layers do not simultaneously protect against a different kind of corrosive attack at temperatures between 600.degree. C. and 800.degree. C. As FIG. 1 of the hereinafter-described drawing shows, and according to tests which have become known in the interim, there are, in fact, two different types of attack or aggression for high-temperature corrosion.
FIG. 1 shows that, in addition to the aforementioned high-temperature corrosion within a range of approximately 850.degree. C. (hereinafter referred to as HTCI), for which heretoforeknown protective layers have been formed, another strong corrosion mechanism exists which has its maximum within a range of approximately 700.degree. C. FIG. 1 is a plot diagram of the corrosion rate against temperature.
In certain types of operation of gas turbine installations, especially in cases wherein the turbine operates in a partial-load region for relatively long periods of time, the corrosion mechanism at 700.degree. C. (hereinafter HTCII) plays a decisive role in the service life of components. It has in fact been found that this type of corrosion in partial-load operation gradually destroys the protective layers intended to protect against attacks at higher temperatures, so that, during later full-load operation at an even higher temperature, the components are exposed, unprotected, to the other attack mechanisms.
In German Published Prosecuted Application (DE-A) 31 04 581, reference has already been made to the additional problem of corrosion at lower temperatures in gas turbines. A solution to this problem which is proposed therein is to apply additionally a silicon-enriched layer on the outside of a layer forming aluminide which is corrosion-resistant at high temperatures, in order to improve the resistance to corrosion attacks at average or medium temperature. Such a construction is not suited for all applications, with respect to temperature distribution in gas-turbine component members.